Habitat Selection Behavior Of Predatory Ladybeetles In Agroecosystem Of Northern China

In many agricultural systems, predatory ladybeetles play an important role in the decreased population growth rates of aphids and reductions in peak aphid densities. Also, the studies relating to the habitat searching behavior of predatory ladybeetles were just confined to the relationship between individual ladybeetle species and corresponding individual aphid species. However, the habitat searching and preferring behavior of predatory ladybeetles in agroecosystem with the coexisting of diversity crops remains unclear. In the present study, we examined the relationship between the number of prey aphids, sugar source, plant refuge and the population abundance of three ladybeetle species through systematic investigation in commercial fields and field plots during 2013-2014. The main results were summarized as follows.In commercial fields and field plots, the dominant ladybeetle species were Harmonia axyridis Pallas and Propylea japonica Thunberg, and Coccinella septempunctata L. was ranked as less common species. We analysed the relationship between aphid abundance and that of ladybeetle complex population. The results show that, in commercial fields, the abundance of aphid populations were positively correlated the abundances of ladybeetle complex population, the adults, the larvae during 2013-2014 (P< 0.05). More specifically, aphid abundance were also positively correlated with the abundances of H. axyridis, its adults and larvae, and the relationships between aphid abundance and the abundance of P. japonica, its adults, larvae were similarly positively correlative in both years (P< 0.05). The relationship between the abundance of aphid were positively correlative with those of C. septempunctata and its larvae in 2014 (P< 0.05), whereas not for that between aphid and adult C. septempunctata in 2014 and all relationship between aphid and C. septempunctata in 2013 (P> 0.05). As for field plots, significantly positive correlation was found between the abundances of aphid and ladybeetle larvae in 2013 (P< 0.05), whereas no significant relationship was determined for ladybeetle complex population and their adults (P> 0.05). During 2014, aphid abundance were positively correlated with the abundance of ladybeetle complex population (P< 0.05). For H. axyridis, all relationship with aphid abundance were similar to ladybeetle complex population in 2013 and 2014. For P. japonica, in addition to the similarly significant correlation found for H. axyridis, the aphid abundance was also positive correlation with total P. japonica population in 2014. For C. septempunctata, except for its total population and its larvae in 2013, no significant relationship between aphid and C. septempunctata abundances was found within these two years. These findings demonstrated that prey aphids abundance was the determinants of habitat selection and population growth of ladybeetles in agroecosystem of northern China.Meanwhile, based upon plant species occurrence and distribution, as well as aphids abundance on crops in northern China, the transfer of ladybeetles in commercial fields were determined during three successive periods. From early-May to early-June, ladybeetles mainly stay on Triticum aestivum L., Prunus persica L., Malus pumila Mill., Descurainia Sophia (L.) Webb. ex Prantl, Hemistepta lyrata Bunge and Populus tomentosa Carr. (seedling), etc. While from middle-June to early-August, Gossypium hirsutum L., Vitis vinifera L., Artemisia lavandulaefolia DC., A. annua L. and P. tomentosa (seedling), etc. were the dominate habitats of ladybeetles. In the last periods, i.e., from middle-August to early-September, ladybeetles move to Zea mays L., A. lavandulaefolia, Chenopodium album L. and Ulmus pumila L., etc.Furthermore, we examined the influence of plant growth stage on the population dynamics of ladybeetles. The results indicated that even if the population abundance of aphids were very low or become to none on Sophora japonica L., Vigna radiata (L.) Wilczek, Ricinus communis L., Impatiens balsamina L. and Cosmos sulphureus Cav. during the flowering growth stage, there are still higher population abundance of ladybeetles. These results demonstrated that sugar source play a certain extent role in the habitat searching and preferring behavior of predatory ladybeetles. In addition, although there are no aphids on Zea mays L. and Sorghum vulgare Pers. during the period of flare opening, a plentiful of ladybeetles can be surveyed, this is because of this special growth stage of Z. mays and S. vulgare can afford the refuge that supplied the moisture microclimate thereby resisted the hot summer. It proved that ladybeetles show habitat preferring behavior to some crops that can offer plant refuge for them.Our work will enhance our understanding of the movement and dispersal of ladybeetles among different habitats and plant species. It also provides insight into the mechanism of habitat searching and preferring behavior of predatory ladybeetles in the agroecosystem, as well as the construction the plant-mediated support system for natural enemies, and the promotion of the regional conservation and pest control of natural enemies in northern China.